How to read UDP packet payload using netif_receive_skb tracepoint with the libbpf framework

[hundredmiles]

Asked in stackover flow as well and posting it here with a hope of a faster reply

I am trying to retrieve the payload of UDP packets using tracepoint netif_receive_skb at the receiver. The goal is to get the packet payload and send it to userspace for further processing.

Method 1:
I am navigating to the payload with the following code but, when I print and check the payload, some part of the data at the end is missing.

unsigned char *payload = (unsigned char *)(udph + 1);
bpf_printk("%s", payload);

Method 2:
Get the payload data in the custom structure and send it out using event. But I receive the error R8 invalid mem access 'inv'. Here is the code snippet.
:

struct custom_data
{
	__u32 val1;
	__u32 val2;
	__u32 val3;
	__u32 val4;
};


SEC("tp/net/netif_receive_skb")
int net_netif_receive_skb(struct trace_event_raw_consume_skb *args) 
{
    struct sk_buff *skb = (struct sk_buff *)BPF_CORE_READ(args, skbaddr);

    // packet parsing steps

    // Values of end and tail from skb structure are less than the values of data and head from the structure. May be due to non-linear data (explanation would be helpful for this case) and so, using data and len members from the skb structure for the data_end bound.

    unsigned char *data_end = BPF_CORE_READ(skb, data) + BPF_CORE_READ(skb, len);

	if ((unsigned char *)(udph + 1) > data_end)
		return -1;
	
    struct custom_data *custom_data= (struct custom_data *)(udph + 1);
    
	if ((unsigned char *)(custom_data + 1) > data_end)
		return -1;

    __u32 val1 = custom_data ->val1;
    bpf_printk("port: %d", bpf_htons(port2_real_dst));
    // when the bpf_printk() line is commented out, the code is loaded fine!

Error from the verifier

; unsigned char *data_end = BPF_CORE_READ(skb, data) + BPF_CORE_READ(skb, len);
189: (61) r1 = *(u32 *)(r10 -32)
; unsigned char *data_end = BPF_CORE_READ(skb, data) + BPF_CORE_READ(skb, len);
190: (0f) r8 += r1
; if ((unsigned char *)(udph + 1) > data_end)
191: (3d) if r8 >= r7 goto pc+124

from 191 to 316: frame1: R0_w=invP(id=0) R1_w=invP(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R6_w=invP(id=22) R7=invP(id=21) R8_w=invP(id=0) R9=invP61 R10=fp0 fp-32=mmmmmmmm fp-40=00000000 fp-48=00000000 fp-56=000mmmmm fp-64=mmmmmmmm fp-72=00000000 fp-80=mmmmmmmm fp-88=invP61
; if ((unsigned char *)(telemetry_data + 1) > data_end)
316: (bf) r1 = r7
317: (07) r1 += 28
; if ((unsigned char *)(telemetry_data + 1) > data_end)
318: (3d) if r8 >= r1 goto pc+71

from 318 to 390: frame1: R0=invP(id=0) R1=invP(id=0) R6=invP(id=22) R7=invP(id=21) R8=invP(id=0) R9=invP61 R10=fp0 fp-32=mmmmmmmm fp-40=00000000 fp-48=00000000 fp-56=000mmmmm fp-64=mmmmmmmm fp-72=00000000 fp-80=mmmmmmmm fp-88=invP61
; __u16 port2_real_dst  = telemetry_data->port2_real_dst;
390: (71) r1 = *(u8 *)(r7 +26)
R7 invalid mem access 'inv'

Method 3:
Read n bytes from the kernel memory using bpf_probe_read_kernel(data, sizeof(*custom_data), udph + 1); retrieves only part of the data with garbage values.

Am I missing any obvious steps? Any other methods using libbpf library to read udp packet payloads.

I am using kernel version 4.19